@leocg Thanks. I am making some progress, as a result of you pointing me to the Settings item which I had not found before. I now get the Chromecast icon displaying with a video but although clicking produces a 'play to TV' notice it has not worked so far.
I get part of a notice popping up at the top of the screen but it is inert and as I cannot see it all I do not know if there is, for eg., a button to click to activate Casting ---it is not activated.
On the TV when I select Chromecast there is a message to say that it cannot find the required WiFi connection,, despite the fact that it is displaying the very thing it cannot find and doing so correctly, with the right name, PIn etc. This may be an issue with my otherwise functioning WiFi rather than Opera, so I will try Chrome which I do not normally use.
Also, I have not been able to get the Chromecast icon to appear when trying to cast images --- it used to be so simple in comparison.